1. Field of the Invention
The present invention relates to the compensation of a video image to provide an accurate black-and-white reproduction of the tonal luminance differences of the image.
2. Description of the Prior Art
At the present time, in a number of fields, a photograph is made from a video image, or the video image is part of the image reproduction chain, and it is desired that the photograph be an accurate reproduction of the tonal luminance differences of the image. For example, in the medical field a CAT X-ray scan, an ultrasonic scan, an NMR scan (nuclear magnetic resonance) or thermograph image is produced on a video monitor CRT (cathode ray tube) screen for immediate viewing by the physician. Simultaneously the same image is produced on another video CRT screen which is part of an electronic camera photographic system. That electronic camera photographic system includes the CRT tube, the video electronic system to produce the image on the CRT screen, optical lenses and equipment to focus the image and allow accurate exposure, and a photographic sensitive material to take a photograph of the image which is on the CRT screen.
The photograph is taken on photosensitive film that is developed using conventional black-and-white (or color) film development processes. The developed film may be directly viewed by the physician using a light box which illuminates the film. Such light boxes are often non-uniform in their illumination across the film and differ greatly in illumination from one box to another.
In the printing field it is sometimes desired to make an accurate printed picture from the image on a video monitor screen. For example, the screen may show a computer-generated image. That image is reproduced on a photosensitive material or directly on a printing plate to print a hard copy that should be similar in tone, luminance and color to the original object or scene.
It has been found that a photographic image reproduction may vary significantly from the original image on the video monitor screen. Some of the distortions are due to the inaccuracy of the CRT screen and video image reproduction process. That type of distortion has been recognized and compensation methods have been suggested, generally dealing with the problem as it affects an entire group of video monitors. Similarly, the distortions due to the camera have been recognized and treated, generally by improvements in the optics of the camera or overall corrections in video components (brightness, contrast, etc.). However, many of the distortions are not predictable and may vary from day to day and from one device to another.
Set forth below is a discussion of the problems most frequently encountered in producing an accurate picture in the video-to-photographic process as it relates to accurate tonal black-white reproduction. The contribution of each problem to the total final distortion of the picture can change in its characteristics periodically and is not predictable. One-day film development bath temperature may be incorrect and seriously distort the picture, and the next day it may still be incorrect but have only a minor adverse effect due to partial compensation distortions from other components in the system. In addition, most of the problem-causing effects are non-linear, so that complete compensating for them in a simple direct way is impossible.
The problems with the conventional system are explained in connection with FIG. 1, which is a block diagram of a conventional black-white photographic system. As shown in FIG. 1, the video image is produced by the video source 10, which may be a video camera, a computer graphics output, or a VCR. The video signal is viewed directly on the monitor CRT screen 11. The same video image is shown on an internal CRT screen 13 in the electronic camera 14. Generally the image on screen 13 is a negative image compared to the image on monitor screen 11. The camera 14 includes an optical system to take a still black-white or color photograph on the film 15 which is removed from the camera 14, after the series of photographs is taken and developed in a film processor 16. The film may itself be the final hard copy 17 or may be used to produce a black-white print using conventional print processing methods.
Each step of this conventional process gives rise to unpredictable distortions. The first set of distortions arises in the CRT device, and its screen 13, which is part of the electronic camera 14. The ratio between luminance values, i.e., the ratio between shades of gray, on the screen 13 may be inaccurate. For example, the CRT tube may be unevenly coated with phosphor, or may be aged or may be subject to flare. In addition, the relationship of the signal voltage applied to produce a certain brightness is not linear. Consequently, the negative image produced on the screen of the electronic camera may not be directly proportional, i.e., accurate, compared to the positive image on the monitor screen. A detailed description of the inaccuracy of an electronic camera due to CRT distortions is found in Schwenker, R. P., "Film Selection Considerations For Computed Tomography and Ultrasound Video Photography": Proc. SPIE--Appl. of Optical Instrumentation In Medicine, VII, 1979; 173, pgs. 75-80.
The electronic camera takes a picture using conventional black-white photo-sensitive film. The film density, in such film, does not accurately reproduce the differences in the gray scale because the film has a non-linear "characteristic curve" of density against log exposure. The exact shape of the curve varies from one manufacturer to another and even from one batch of film to another, see The Theory of Photo Process, T. H. James, pgs. 501-505, 4th Edition, Macmillan.
When the film is developed, distortions may arise from the variability of the process chemistry, variations in process temperature, variations in the film, and the non-linear characteristic of the photosensitive material of the film. In those cases in which the film is duplicated or made into a print, additional distortions may occur.
If the developed film is made into a print using a printer, still other distortions may arise from the dot size of the printing, the spread of the dots and the variable absorption of the ink into the paper due to various types and batches of paper. Also, the perception of the gray scale may differ depending on the type of printing process that is used.
After the film or other hard copy is produced, it is viewed under conditions which may detract from the accuracy of the gray scale tones. For example, the film may be placed on a light box whose intensity of illumination is greater at its center than at its sides. Another cause of viewing distortion is the "flare factor" in which flare (non-image light from outside the image) enters the optical viewing system and mainly affects the shadow areas.
The present invention is particularly directed to accurate reproduction of the luminance differences in value (differences in a gray scale) and absolute luminance on a black-white video screen. However, in its broader aspects, the invention is also applicable to the accurate reproduction of color images. The invention is directly applicable to color images in the sense that the video screen may be a color CRT screen and the invention will correct for gray scale distortions in reproducing the image on the color screen. In addition, the reproduction of color images has its own set of problems and distortions, aside from black-and-white tonal differences. These color distortions can also be corrected, and their correction will be discussed at the end of the detailed description.
These color and luminance distortions include (i) that the original color is not exactly matched to the phosphors on the CRT screen so that the color on the screen does not match the original color, (ii) that the color of the photo-sensitive dyes of the film do not match the color on the CRT screen and do not compensate for the color mis-match of the screen phosphors, (iii) that the color of the color photo-sensitive papers, dyes or printing inks do not match the color of the film. In addition, the chemistry for color films and color prints is more complex, and more temperature sensitive, than for black-white film and prints, so that variations in the chemistry or temperature cause distortion shifts in the color. A further problem with color, not found in black-white images, is that the perception of color of the object or video screen (by the human eye differs from the actual color on the film or print.
In U.S. Pat. No. 4,263,001 entitled "Apparatus and Method For Enhancement of Optical Images", in one embodiment, which is not claimed, a video camera is connected to an electronic image modification device which, in turn, is connected to a single frame storage, to prevent feedback, and a monitor CRT.
In U.S. Pat. Nos. 4,492,987 and 4,520,403, both entitled "Processor For Enhancing Video Signals For Photographic Reproduction", the screen of an electronic camera is electronically modified to enhance photographic reproduction. The entire screen is treated as a unit and its brightness or color is changed in accordance with the distortion introduced by a selected photographic film.
In U.S. Pat. No. 4,658,286 entitled "Method and Apparatus For Correcting Distortions In Reproducing Systems", a type of feedback system is described. In one embodiment three photocells look at a corner of the CRT screen having test colors and their outputs are compared to reference colors.